The ability to strengthen glass sheets allows glass to be used in applications, such as cover plates for cellular phones and other mobile electronic applications, where everyday exposure and use require high strength and scratch resistance.
The induction of surface compressive stress profiles in glass is a well known approach for strengthening. Ion exchange, or chemical strengthening, of glass can be used to incorporate residual stresses in the glass. In the ion exchange process, a glass containing alkali metal ions is exposed to a molten salt bath that contains alkali metal ions which are larger than those ions that are initially in the glass. As a result of chemical potential differences, some alkali metal ions in the glass are replaced by the larger ions, thus creating a layer of larger volume. This larger volume layer leads to the development of compressive stress in the surface of the glass and, consequently, a compensating tensile stress in the interior of the glass to maintain the equilibrium of forces. Since glass only fails under tension, this compression layer strengthens the glass. If the internal tensile stress generated by the exchange layer is critically high, the glass may be frangible—i.e., the glass may suffer substantial fragmentation upon damage or spontaneous breakup. The thinner the glass, the higher the tensile stress developed for a given depth of layer.
Chemically strengthened soda-lime type glasses are commonly used as cover plates or windows mobile electronic devices such as mobile or cellular telephones, pagers, audio and video players, and games. Soda-lime glasses are difficult to chemically strengthen, and usually require long treatment times to achieve adequate strength by ion exchange. Soda-lime glass sheets are commercially fabricated via floatation in a tin bath, and these glasses cannot be formed by methods such as down-draw processes that are associated with the manufacture of precision sheet glass.